1. Field of the Invention
The present invention relates to the art of coating glass with metals and more particularly to a process for forming a reflecting copper coating on a face of a glass substrate.
The invention may prove most advantageous in the production of vacuum flasks, in electronics, as well as in the production of translucent glass, and colored filters.
A coating technique currently in wide use in the production of vacuum flasks is that of silvering.
To form the above-mentioned coatings, however, a precious metal, namely silver, is to be used, the world reserves of which are limited. It seems reasonable therefore to make use of base metals when forming reflecting coatings. Specifically, copper coatings are of practical significance. Yet the conventional processes for forming metal coatings on a face of a glass substrate do not provide copper coatings which are sufficiently reflective. In solving this problem a copper coating formed by reducing copper from aqueous solutions of copper salts is of practical significance.
2. Description of the Prior Art
Known in the art is a process for forming a copper coating on a face of a glass substrate by reducing the metal from aqueous solutions of copper salts (M. Shalkauskas, V. Vashkyami, Khimicheskaya metallizatsia plastmass (Chemical coating of plastics with metals), Leningrad, "Khimiya" publishers, 1977, pp. 51-65 and 95-97) comprising the following steps:
activating the glass face, which is carried out in two stages of which the first one (sensitization) consists in treating the glass face with a sensitizer, namely an aqueous solution containing
______________________________________ SnCl.sub.2.2H.sub.2 O 40 to 50 g/l 35% HCl 40 to 50 ml/l, ______________________________________
and after the treatment the thus sensitized glass face is washed with water and dried; the second stage (activation proper) consists in treating the washed and dried sensitized face with an aqueous solution of an activator, heated to 50.degree. to 60.degree. C. and containing
______________________________________ AgNO.sub.3 2 to 90 g/l 25% NH.sub.3 10 to 100 ml/l, and ______________________________________
and then the thus activated face is dried; and
chemically coating the activated face of a glass substrate for 20 to 25 minutes in a coppering solution at an ambient temperature ranging from 70.degree. to 80.degree. C.;
drying the copper coated glass face.
The best of the conventional coppering solutions is that which is prepared by mixing the following two solutions:
______________________________________ solution I copper sulphate 13 g nickel chloride 4 g formalin 50 ml water 1000 ml solution II sodium hydroxide 10 g Rochelle salt 43 g sodium carbonate 4 g water 1000 ml ______________________________________
(See B. Ya. Kaznachey, Galvanoplastika v promyshlennosti, Gosizdatmestprom RSFSR, Moscow, 1955, p.56).
Sensitization of the glass face with the above-mentioned sensitizer comprising from 40 to 50 g/l of SnCl.sub.2.2H.sub.2 O requires that a sensitized face be washed with water whereby hydrolysis of SnCl.sub.2.2H.sub.2 O occurs, which is a critical requirement for sensitization of a glass face to be effected.
A great amount of the hydrolysis products occurring on the glass face dissolve and by reducing silver ions in the process of activation form slime in the activator solution.
Moreover, sensitization of the glass face with the above-mentioned sensitizer brings about a considerable dependence of structure and amount of the adsorbed products on the shape of the face. When surfaces similar to vacuum flasks and the like are washed, the acid present in the sensitizer is not washed away at a time from different portions of the surface. This results in uneven deposition of silver particles onto the surface in carrying out the step of activation and hence an uneven copper coating is formed on this surface, thus making it impossible to produce a reflecting copper coating and affecting its adhesion to the surface being coated.
To make the activation process more effective it has been proposed that an aqueous solution of an activator containing 2 to 90 g/l of AgNO.sub.3 and 10 to 100 ml/l of 25% NH.sub.3 be heated to a temperature in the range from 50.degree. to 60.degree. C.
However a hot solution of the activation impairs adhesion of the copper coating to the glass surface coated by a chemical process. And again, ammonia present in the solution of the activator deteriorates the quality of the copper coating being produced.
Furthermore, the activation step carried out on a sensitized surface in the above solution of the activator containing substantial quantities of AgNO.sub.3 requires that the sensitized surface be dried to improve adhesion thereof with silver particles.
It is also to be noted that such a solution of the activator that comprises a considerable amount of AgNO.sub.3 cannot be used repeatedly since the content of AgNO.sub.3 will undergo a relatively speedy change due to loss of finely-divided silver particles, which results in an unstable activation of the glass surface.
It is also worth consideration that the step of chemically coating the glass face is carried out at an ambient temperature of 70.degree. to 80.degree. C. thus requiring extra equipment for maintaining such temperature.